Copper and its metal alloys have been used for thousands of years. The importance of copper, as well as a variety of other metals, has led to a continuing search for more efficient and productive procurement methods. One method of copper extraction is a process of leaching, coupled together with solvent extraction, and finally copper production by electrowinning. Leaching is typically carried out by stacking the ore in piles on a prepared pad or by stacking it in a small canyon. A solution of sulfuric acid is then applied, and as the acid solution is trickled down through the heap, copper is dissolved from the rock. The resultant copper-bearing solution (pregnant leach solution or PLS) is collected, and then transferred to the solvent extraction plant, where it is contacted by vigorous mixing with an organic solution comprising an extractant dissolved in a kerosene-like hydrocarbon diluent. In this extraction, the copper (as cupric ion) is transferred to the organic phase, where it forms a chelate-type complex with the extractant. After contact, the mixture of aqueous and organic is allowed to separate. The copper-depleted aqueous solution (raffinate) exits the solvent extraction plant, and the organic is transferred to stripping, where it is contacted with a strong acid solution. In stripping, the cupric ion is transferred to the aqueous phase and protons are transferred to the organic. The now copper-depleted organic is returned to extraction for re-use. The copper-rich aqueous strip solution (pregnant or rich electrolyte) is transferred to electrowinning. In electrowinning, copper is plated as metal from solution at the cathode, and water is broken down at the anode to form oxygen and protons as acid. Depending on the climatic conditions at the site, the size of the ore heap or dump, and the irrigation rate, the temperature of the PLS entering the plant could range from about 10° C. to about 30° C. As a result, temperatures in extraction typically range from about 20 to 25° C., and temperatures in stripping may range from about 30 to 35° C. The temperature in the electrowinning cells is typically about 45° C., incorporated herein by reference. This acid leach process may also be used for other metals. Additionally, leaching with ammonia may be carried out analogously. Combinations of ammonia with an ammonium salt, such as ammonium carbonate or ammonium sulfate, have been used on a commercial scale to leach copper metal (recycling applications), copper oxide ores and copper sulfide ores. Ammonia leaching can be applied other metals such as nickel and zinc as well.
Reagents useful in such processes should generally possess certain qualities. Examples of important features are the rates of reaction, phase separation and reagent stability. A detailed discussion of the useful characteristics in a liquid ion exchange reagent is available in Swanson, “Liquid Ion Exchange: Organic Molecules for Hydrometallurgy” presented at the International Solvent Exchange Conference September 1977.
Several extractant reagents have been used, including some phenolic oxime extractants. Among those used are 5-nonylsalicylaldoxime, 5-nonyl-2-hydroxyacetophenone oxime and 5-dodecylsalicylaldoxime. However under certain conditions of use, the current reagents are not ideal and have had issues not yet fully addressed. For example, these aldoximes bind copper very tightly, and only a small part of the copper can be recovered in stripping under the commercially typical conditions of acid and copper content in the lean electrolyte that is used as strip media. To maximize stripping, one typically adds a thermodynamic modifier to the extractant. Alternatively, extractants can be formulated that have different relative extractant strengths, which strip significantly better than the standard aldoximes by themselves. Blends of aldoximes and ketoximes have been used, and demonstrate that ketoximes act as an extractant, as well as a thermodynamic modifier. However, the copper content on the stripped organic is lower than one would expect based on consideration of the stripping behavior of the individual oximes.
Another general problem is extractant loss (also known as degradation) via chemical hydrolysis to the corresponding ketone or aldehyde. The concentration of the hydrolysis products in the organic phase increases until the rate of formation equals the rate of loss in entrainment. The rate at which hydrolysis occurs is dependent on the acid concentration and the temperature of the system. Current reagents may not work properly due to hydrolysis. One trend in the industry is towards the treatment of primary copper sulfide concentrates by hydrometallurgical routes rather than smelting. These processes result in the production of leach solutions which are very warm. Solutions fed to the copper solvent extraction process will range in temperatures from about 35° C. to 50° C., or higher. Higher temperatures also occur when the oxide ores are extremely rich, such as the ores from the Democratic Republic of the Congo. They are typically vat or agitation leached with sulfuric acid. The leaching reactions are quite exothermic, resulting in PLS for extraction that are higher in temperature than typical heap or dump leach operations. The higher temperature results in a significantly higher rate of hydrolysis of the oxime extractants. This leads to buildup of the hydrolysis products in the circuit organics to very high levels relative to that observed in typical head and dump leach operations. Due to the higher rate of degradation, the level of degradation products can approach levels as high as 100% of the oxime concentration in the circuit organics. This results in a significant increase in the density and viscosity of the organic phase, which in turn is reflected in slower phase disengagement and higher entrainments.
Another problem with current technology is with copper selectivity over iron. Copper/iron selectivity is very important for some solvent extraction/electrowinning systems. Iron that is transferred to the electrowinning system has a negative effect on the processing of copper in the electrolyte. As the concentration of ferric ions increases, there is a substantial drop in current efficiency. In addition to the cost incurred by the drop in current efficiency, there is the additional cost of bleeding the system to control the iron concentration. Bleeding electrolyte results in the reduction of cobalt concentration (in addition to other additives) which is added to protect lead anodes, and this can be a large expense in an electrowinning plant.
Current reagent technology could also be improved when nitrate is present in the PLS or strip solution. Nitrate in the PLS or strip solution can lead to attack on the phenolic oximes resulting in nitration of the ring to form the corresponding 3-nitro aldoximes or ketoxime. The nitro oximes are extremely strong copper chelators. They cannot be stripped under typical plant conditions resulting in loss of net transfer. Such problems are discussed in the Virnig, et al., “Effects of nitrate on copper SX circuits: A case study” in Proceedings Copper 2003-Cobre 2003, Vol VI-Hydrometallurgy of Copper (Book1), edited by P. A. Riveros, D. Dixon, D. B. Dreisinger, J. Menacho; Canadian Institute of Mining, Metallurgy and Petroleum; Montreal, Quebec, Canada; 2003, pp 795-810. There have been attempts to deal with this nitration issue. For example, it has been proposed to add lower molecular weight phenol to the extractant formulation as a sacrificial lamb. The phenol is more readily nitrated than is the oxime, and so long as there is any phenol present, the oxime is protected. However, as soon as the phenol is consumed, then nitration of the oxime will occur.
Yet another problem relates to currently used oximes for extraction of copper and nickel from ammoniacal solutions. In applications involving extraction of nickel or copper from ammonia, one typically finds that degradation of the organic by hydrolysis of the oxime is an issue. During such extraction, the resultant complex carries with it some chemically bound ammonia, which is undesirable. The ammonia is transferred to stripping where it consumes acid to form the corresponding ammonium salt which builds up overtime in the circulating electrolyte and can lead to the formation of insoluble salts such as nickel ammonium sulfate which can cause plugging of lines, etc. There is thus a need for reagents and/or methods that address one or more of these problems.